Venous thromboembolism (VTE) is a major national health problem and the recent Surgeon General's Call to Action identified key gaps in knowledge which we will specifically address. Our specific aims are in Aim 1 determine why VTE remains such a persistent problem by updating the Olmsted County, MN VTE inception cohort to include the 45-year period, 1966-2010, and 1a) test whether recently implemented CMS performance measures have affected secular trends in VTE incidence and prophylaxis- related complications, 1b) quantify changes over time in the population-attributable risk of major VTE risk factors, and 1c) determine whether changes in the prevalence of major VTE risk factors can account for trends in the observed incidence of VTE; in Aim 2 to determine when genetic testing is appropriate by 2a) testing Factor V Leiden, Prothrombin G20210A, and novel ABO SNPs as risk factors for VTE after hospitalization for major surgery and acute medical illness, and 2b) developing and 2c) validating VTE risk assessment tools for these two high-risk populations; in Aim 3 to conduct research into how [arm] DVT should best be managed by identifying all Olmsted County residents with incident arm cerebral, hepatic, portal, splenic, mesenteric and renal vein thrombosis and 3a) estimate the incidence of thrombosis in these other venous circulations, 3b) quantify outcomes (survival and VTE recurrence), and 3c) test other venous circulation thrombosis as potential predictors of survival and VTE recurrence; and in Aim 4 to investigate the roles of IVC filters by identifying all Olmsted Count residents with IVC filter placement over the 45-year period, 1966-2010, and 4a) estimate the incidence of IVC interruption or filter placement, 4b) determine outcomes (survival, complications and VTE incidence and recurrence), and 4c) test IVC filter placement as a risk factor for incident VTE, or as predictors of survival and recurrent VTE.